Aqua-ions water exchange

The labilities of the alkali metals are close to the diffusion-controlled limit. Rate constants increase by approximately one order of magnitude, the increase in lability from Li+ to Cs+ being dominated by a decrease in surface charge density and an increase in coordination number which changes from values of 4-6 for Li+ to 8 for Cs+, with intermediate values for Na+, K+, and Rb+. The residence time for a coordinated water on the Cs+(aq) ion of <1 ns makes its behavior closely similar to that of Eu2+(aq), where these are the two most rapidly exchanging aqua ions. 

The simplest reactions to study, those of coordination complexes with solvent, are used to classify metal ions as labile or inert. Factors affecting metal ion lability include size, charge, electron configuration, and coordination number. Solvents can by classified as to their size, polarity, and the nature of the donor atom. Using the water exchange reaction for the aqua ion [M(H20) ]m+, metal ions are divided by Cotton, Wilkinson, and Gaus7 into four classes ... 

Table 1.9 Rate Constants for Water Exchange in Metal Aqua Ions...

Fig. 4. Interpretation of volumes of activation (in cm3 mol-1) for water exchange on aqueous M(H20)g+ in terms of contributions from bond-making and -breaking (a) summary of volumes of activation for metal aqua ions (b) calculated curves for Al(III), Ga(III), and In(III) with use of the Connolly volumes in Table IV.

Rate Constants and Activation Parameters for Water Exchange on Lanthanide Aqua Ions... 

EXAFS study on Eu2+ and Sr2+ in both solid state and aqueous solution gave coordination numbers of 8.0 for strontium(II) and 7.2 for europium(II) (228). The water exchange rate measured on the divalent europium aqua ion is the fastest ever measured by 170 NMR (Table XVI) (2). The activation volume is much more negative (—11.7 cm3 mol-1) than those determined on trivalent lanthanide aqua ions clearly indicating an a-activation mechanism which is most probably a limiting... 

Enormous acceleration of substitution at ruthenium(II) can be obtained by appropriate choice of ligands. Following the demonstration of remarkably rapid water exchange and complex formation (7d or D in mechanism) at the organometallic aqua-ion [Ru(ri5-C5Me5)(H20)3]2+ (150), comparably rapid substitution has been demonstrated at... 

In the Re(V) and W(IV) aqua oxo complexes, comparison of both the complex formation of the [MO(OH2)(CN)4], by NCS ions and the water exchange (k iq) shows a relative increase in reactivity of approximately 3 orders of magnitude (Table II), which is in direct agreement with the previously (1, 2, 50) concluded dissociative mechanism. The increase in Lewis acidity of the Re(V) center compared to that of W(IV) is expected to result in a much less reactive system in a dissociative activated mode. 

The resultant aqua ion then racemizes to zero rotation more slowly and without loss of X . In many respects then, the aqua ion is the most suitable one to examine for the relationship of isomerization, racemization, and substitution (using water exchange). For the interconversion... 

The NMRD profile of chromium(III) aqua ion (Fig. 18) is characterized by slow exchanging water protons, as clearly shown by the fact that the solvent proton relaxivity at low fields increases with increasing the temperature. The occurrence of slow exchange hinders any increase in relaxivity below 300 K, thus explaining the fact that the contact dispersion disappears in the low temperature profiles, whereas it is well shown in the high temperature profiles, as already discussed in Section I.C.8. 

The smaller contribution to solvent proton relaxation due to the slow exchanging regime also allows detection of second and outer sphere contributions (62). In fact outer-sphere and/or second sphere protons contribute less than 5% of proton relaxivity for the highest temperature profile, and to about 30% for the lowest temperature profile. The fact that they affect differently the profiles acquired at different temperature influences the best-fit values of all parameters with respect to the values obtained without including outer and second sphere contributions, and not only the value of the first sphere proton-metal ion distance (as it usually happens for the other metal aqua ions). A simultaneous fit of longitudinal and transverse relaxation rates provides the values of the distance of the 12 water protons from the metal ion (2.71 A), of the transient ZFS (0.11 cm ), of the correlation time for electron relaxation (about 2 x 10 s at room temperature), of the reorienta-tional time (about 70 x 10 s at room temperature), of the lifetime (about 7 x 10 s at room temperature), of the constant of contact interaction (2.1 MHz). A second coordination sphere was considered with 26 fast exchanging water protons at 4.5 A from the metal ion (99), and the distance of closest approach was fixed in the range between 5.5 and 6.5 A. 

A. Water exchange on trivalent aqua-ions of lanthanides... 

To study water exchange on aqua metal ions with very slow exchange of water molecules, an isotopic labeling technique using oxygen-17 can be used. A necessary condition for the applicability of this technique is that the life time, tm, of a water molecule in the first coordination shell of the ion is much longer that the time needed to acquire the 0-NMR spectrum. With modern NMR spectrometers and using enrichments up to 40% in the acquisition time can be as short as 1 s. 

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